The proposed research will provide information about the electrical activity and neuroanatomy of certain neurosecretory neurons in the brain of the tobacco hornworm, Manduca sexta. These neurons contain prothoracicotropic hormone (PTTH), which controls the events of larval molting, metamorphosis of larva to pupa, and development of the pupa to the adult insect. Thus, the neurons under study are thought to be central components in the control of insect development. Using extracellular and intracellular recording techniques, a search will be made for circadian gating of PTTH-cell electrophysiological activity in pupal brains. The level of spike activity in PTTH cells will be compared in diapausing pupae (programmed by the environmental photoperiod to cease development) and in pupae which develop normally. Additionally, the dendritic pattern (structure of fine neuronal processes) of these cells from developing and diapausing animals of different stages will be compared using cobalt-fill plus silver intensifications techniques. These techniques will also be used to trace the specific axonal projections of PTTH cells within the neurohaemal organs, for possible development of a chronic recording method. This research should increase our understanding of one major aspect of the brain-centered control of insect development. Additionally, it is possible that detailed electrophysiological and neuroanatomical studies of this system may lead to new insights about the basis of long-term functional changes within nervous systems.